The quadrupole ion trap (QIT) was first disclosed in the year 1952 in a paper by Paul, et al. This paper disclosed the QIT and the disclosure of a slightly different device which was called a quadrupole mass spectrometer (QMS). The quadrupole mass spectrometer was very different from all earlier mass spectrometers because it did not require the use of a magnet and because it employed radio frequency fields for enabling the separation of ions, i.e. performing mass analysis. Mass spectrometers are devices for making precise determination of the constituents of a material by providing separations of all the different masses in a sample according to their mass to charge ratio. The material to be analyzed is first disassociated/fragmented into ions which are charged atoms or molecularly bound group of atoms.
The principle of the quadrupole mass spectrometer (QMS) relies on that fact that within a specifically shaped structure, radio frequency (RF) fields can be made to interact with a charged ion so that the resultant force on certain of the ions is a restoring force thereby causing those particles to oscillate about some referenced position. In the quadrupole mass spectrometer, four long parallel electrodes, each having highly a precise hyperbolic cross sections, are connected together electrically. Both dc voltage, U, and RF voltage, V.sub.0 cos .omega.t, can be applied. When an ion is introduced or generated within the spectrometer, if the parameters of the quadrupole are appropriate to maintain the oscillation of those ions, such ions would travel with a constant velocity down the central axis of the electrodes at a constant velocity. Parameters of operation could be adjusted so that ions of selected mass to charge ratio, m/e, could be made to remain stable in the direction of travel while all other ions would be ejected from the axis. This QMS was capable of maintaining restoration forces in two directions only, so it became known as a transmission mass filter. The other device described in the above mentioned Paul, et al. paper has become known as the quadrupole ion trap (QIT). The QIT is capable of providing restoring forces on selected ions in all three directions. This is the reason that it is called a trap. Ions so trapped can be retained for relatively long periods of time which supports separation of masses and enables various important scientific experiments and industrial testing which can not be as conveniently accomplished in other spectrometers.
The QIT was only of laboratory interest until recent years when relatively convenient techniques evolved for use of the QIT in a mass spectrometer application. Specifically, methods are now known for ionizing an unknown sample after the sample was introduced into the QIT (usually by electron bombardment), and adjusting the QIT parameters so that it stores only a selectable range of ions from the sample within the QIT. Then, by linearly changing, i.e., scanning, one of the QIT parameters it became possible to cause consecutive values of m/e of the stored ions to become successively unstable. The final step in a mass spectrometer was to sequentially pass the separated ions which had become unstable into a detector. The detected ion current signal intensity, as a function of the scan parameter, is the mass spectrum of the trapped ions.
U.S. Pat. No. 4,736,101 describes a quadrupole technique for performing an experiment called MS/MS. In U.S. Ser. No. 4,736,101, MS/MS is described as the steps of forming and storing ions having a range of masses in an ion trap, mass selecting among them to select an ion of particular mass to be studied (parent ion), disassociating the parent ion by collisions, and analyzing, i.e. separating and ejecting the fragments (daughter ions) to obtain a mass spectrum of the daughter ions.
In a copending, simultaneously filed patent application, I have disclosed and claimed an improved QIT technique for isolating the parent ion. For purposes of understanding the background for this invention, I have incorporated by reference, the copending application entitled, "Quadrupole Trap Improved Technique of Ion Isolation," Ser. No. 890,990, filed May 29, 1992, now U.S. Pat. No. 5,198,665.
The preferred technique for disassociating the parent ion into daughter ion fragments is called Collision Induced Disassociation (CID). The CID technique is a more gentle form of ionization than electron bombardment and does not create as many fragments. The technique for obtaining collision induced disassociation (CID) to obtain daughter ions employed in U.S. Pat. No. 4,736,101 is to use a second fixed frequency generator connected to the end plates of the QIT which frequency is at the calculated secular frequency of the retained ion being investigated. The secular frequency is the frequency at which the ion is periodically, physically, moving within the RF trapping field.
By providing an excitation field at the secular frequency, the ion absorbs power and the increased translational motion causes more collisions between ions. The collisions induce conversion of translational energy into internal energy and result in a somewhat gentle fragmentation of the ion into major daughter fragments. This is most frequently carried out in the presence of a background gas of lighter mass than the sample to aid in the collision heating process.
The problem with the prior approach of the '101 patent for causing such collisional assisted ionization (CAI) is that the frequency of the supplemental end cap voltage, sometimes called the tickle voltage, cannot be properly determined in advance. Theoretically, the secular frequency of any selected M/e ion is relatively easy to calculate according to the equation W.sub.1 =1/2.beta..sub.z W.sub.0, where W.sub.1 equals the secular frequency, W.sub.0 is the trapping field frequency and .beta..sub.z is a known function of q.sub.z and a.sub.z, as defined by three different equations, depending on the value of q.sub.z, as depicted at page 200 of the text "Quadrupole Storage Mass Spectrometry" by Raymond E. March and Richard J. Hughes, John Wiley & Sons, 1989. However, there are several physical effects which affect the QIT and render it extremely difficult, if not impossible, to determine the precise secular frequency in advance. Specifically, the space charge effect, which depends on the number of trapped ions will shift the stability chart for the trap. Also, slight mechanical errors in the shape of the electrodes and slight variances in the potentials applied to the electrodes can introduce errors which shifts the secular frequency from theoretical values.
Accordingly, it has been necessary to empirically determine the secular resonance frequency for each M/e to be excited. While this step of establishing the specific resonant frequency is possible for known static samples, it can be extremely difficult to accomplish when only small values of sample are available on a dynamic basis, such as is the situation when the sample is the output from a gas chromatograph.
This problem has been previously recognized by Yates and Yost in an article presented during May 1991 and published in the proceeding of the 39th MAS Conference on "Mass Spectroscopy and Allied Topics", entitled, Resonant Excitation for GS/MS/MS in the Quadrupole Ion Trap via Frequency Assignment Prescans and Broadband Excitation," p. 132.
Yates, et al., describes a complex technique for determining the exact secular frequency for CID in an MS/MS experiment involving automatic scanning of the trap with a frequency synthesizer and measuring the absorption as a function of frequency. Since some of the ions are ejected for each scan due to energy absorption, the space charge effects change and it is necessary to employ multiple scans and averaging to correct for this and other instrumental effects. Yates discloses another technique for inducing CID by using a supplemental broadband excitation signal to excite a range of frequencies. The approach in the Yates paper uses an excitation signal that has a bandwidth of approximately 10 KHz. The broadband excitation technique was orally described in the conference, as the application of a synthesized inverse FT time domain waveform to the QIT end caps, where the waveform has a frequency domain representation comprising a band of uniform intensity equally spaced frequencies up to .+-.5 KHz about a center frequency at the calculated theoretical secular frequency.
The problems with this broadband technique is that it has a range of excitation which is wide enough to induce excitation of m(p)+1 ions and of daughter ions that may be formed during the excitation process. Furthermore, the apparatus needed to obtain a tailored, synthesized broad band inverse waveform is expensive and complex.